Larval Myogenesis in the Articulate Brachiopod Argyrotheca cordata (Risso, 1826)

Brachiopoda is a metazoan phylum with a fossil record dating back to

the lower Cambrium and comprises over 12,000 extinct and 400 recent

species. Despite some recent progress using mainly palaeontological and

molecular datasets, the discussion concerning the phylogenetic position

of Brachiopoda remains unsettled. While some classic analyses employing

morphological data assign Brachiopoda to Deuterostomia, recent molecular

data propose a sister group relationship of Spiralia to a clade

Lophophorata that unites Brachiopoda, Ectoprocta, and Phoronida. To

contribute new developmental and micromorphological data to this

debate, we investigated muscle formation in larvae of the brooding articulate

brachiopod Argyrotheca cordata using immunocytochemistry combined

with confocal laser scanning microscopy. Full grown larvae are

three-lobed and show two pairs of bristle bundles. During larval development,

the first anlagen of the musculature develop in the bristle pouches

and the pedicle lobe. Late stage larvae show a network of longitudinal

muscles running from the apical to the pedicle lobe as well as transversal

muscles situated in the apical lobe. Strong muscles attach to both the

bristles and the pedicle lobe. Our work reveals only few similarities

between the larval myoanatomy and myogenesis of A. cordata and the

hitherto investigated phoronid and ectoproct species, which may account

for an early distinct evolutionary pathway of the general ontogeny